First US Bancshares, Inc. (Parent Company Only) Financial Information - Statements of Cash Flow (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Cash flows from operating activities:
Net income	$ 1,045	$ 411	$ 404	$ 847	$ 1,209	$ 1,118	$ 1,005	$ 1,234	$ 2,707	$ 4,566
Adjustments to reconcile net income to net cash provided by operating activities:
Net cash provided by operating activities									6,094	11,315
Cash flows from financing activities:
Dividends paid									(740)	(562)
Treasury stock repurchases									(452)	(1,478)
Net cash provided by (used in) financing activities									97,350	(13,605)
Net increase in cash and cash equivalents									37,385	7,431
Cash and cash equivalents, beginning of period				57,030				49,599	57,030	49,599
Cash and cash equivalents, end of period	94,415				57,030				94,415	57,030
FUSB [Member]
Cash flows from operating activities:
Net income									2,707	4,566
Adjustments to reconcile net income to net cash provided by operating activities:
Distributions in excess of undistributed income of subsidiaries									(1,586)	(2,385)
Change in other assets and liabilities									14	69
Net cash provided by operating activities									1,135	2,250
Cash flows from financing activities:
Dividends paid									(740)	(561)
Treasury stock repurchases									(452)	(1,478)
Net cash provided by (used in) financing activities									(1,192)	(2,039)
Net increase in cash and cash equivalents									(57)	211
Cash and cash equivalents, beginning of period				$ 476				$ 265	476	265
Cash and cash equivalents, end of period	$ 419				$ 476				$ 419	$ 476